U.S. patent application number 13/965255 was filed with the patent office on 2013-12-26 for cholesteric liquid crystal writing tablet erased by a piezoelectric transducer.
This patent application is currently assigned to Kent Displays Incorporated. The applicant listed for this patent is Kent Displays Incorporated. Invention is credited to Andrew DeMiglio, Mark Lightfoot, Duane Marhefka, Oleg Pishnyak, Tod Schneider.
Application Number | 20130342432 13/965255 |
Document ID | / |
Family ID | 49773994 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130342432 |
Kind Code |
A1 |
Schneider; Tod ; et
al. |
December 26, 2013 |
Cholesteric Liquid Crystal Writing Tablet Erased By A Piezoelectric
Transducer
Abstract
A display device includes a cholesteric liquid crystal writing
tablet and a piezoelectric transducer. The piezoelectric transducer
is subjected to a mechanical force that generates a voltage that is
applied to the writing tablet that erases writing on the writing
tablet. A further feature is a display device comprising a
cholesteric liquid crystal display and a piezoelectric transducer.
The piezoelectric transducer is subjected to a mechanical force
that generates a voltage that is applied to the display that places
the display in at least one of a color reflective planar state, a
substantially transparent focal conic state and a gray scale state.
Also featured is a method of erasing a cholesteric liquid crystal
writing tablet.
Inventors: |
Schneider; Tod; (Kent,
OH) ; Pishnyak; Oleg; (Kent, OH) ; DeMiglio;
Andrew; (Cuyahoga Falls, OH) ; Lightfoot; Mark;
(Sterling, OH) ; Marhefka; Duane; (Winona,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kent Displays Incorporated |
kent |
OH |
US |
|
|
Assignee: |
Kent Displays Incorporated
kent
OH
|
Family ID: |
49773994 |
Appl. No.: |
13/965255 |
Filed: |
August 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13927647 |
Jun 26, 2013 |
|
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13965255 |
|
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61664195 |
Jun 26, 2012 |
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Current U.S.
Class: |
345/89 |
Current CPC
Class: |
G09G 3/3493 20130101;
G09G 3/3629 20130101; G02F 1/133553 20130101; G02F 1/13718
20130101; G06F 3/041 20130101; G02F 1/13338 20130101 |
Class at
Publication: |
345/89 |
International
Class: |
G09G 3/34 20060101
G09G003/34 |
Claims
1. A display device comprising a cholesteric liquid crystal writing
tablet, a piezoelectric transducer, and a device that mechanically
applies a force to said piezoelectric transducer that generates a
voltage that is applied to said writing tablet that erases writing
on said writing tablet.
2. The display device of claim 1 comprising coupling circuitry for
the transfer of electrical charge from the piezoelectric transducer
to the writing tablet so as to shape an output signal or minimize
signal loss from said piezoelectric transducer.
3. The display device of claim 2 wherein the coupling circuitry
comprises a diode to rectify an output of said piezoelectric
transducer, providing a voltage substantially of one polarity to
said writing tablet.
4. The display device of claim 2 wherein the coupling circuitry
comprises a resonant inductive and capacitor combination to shape
the output signal of the piezoelectric transducer.
5. The display device of claim 1 comprising electrical conductors
extending from said piezoelectric transducer to display
electrically conductive layers disposed on either side of a
cholesteric liquid crystal layer of said writing tablet, said
voltage being applied along said conductors to said display
electrically conductive layers.
6. The display device of claim 1 wherein said device comprises a
spring loaded striker that applies said force to said piezoelectric
transducer.
7. The display device of claim 1 wherein said piezoelectric
transducer comprises at least one polarized piezoelectric sheet
comprised of piezoelectric polymer and an electrical conducting
layer on each side of the sheet.
8. The display device of claim 7 wherein said piezoelectric polymer
comprises polarized polyvinylidene fluoride or a polarized
copolymer thereof.
9. The display device of claim 7 wherein said device comprises a
snap action switch that applies said force to said piezoelectric
transducer.
10. The display device of claim 1 comprising a permanent electrical
connection between said piezoelectric transducer and said writing
tablet along which said voltage is applied.
11. The display device of claim 1 wherein said display device
includes no power source besides said piezoelectric transducer.
12. The display device of claim 1 wherein said writing tablet is
flexible and includes no rigid housing so as to form liquid crystal
paper, wherein said piezoelectric transducer is integrated with
said writing tablet and said display device includes no power
source besides said piezoelectric transducer.
13. A display device comprising a cholesteric liquid crystal
writing tablet and a piezoelectric transducer, wherein said writing
tablet is flexible and said piezoelectric transducer is laminated
to said writing tablet on a side opposite an imaging side, and
whereby said piezoelectric transducer is subjected to a mechanical
force that is achieved by bending the laminated writing tablet and
piezoelectric transducer together resulting in said piezoelectric
transducer applying a voltage to said writing tablet that erases
writing on said writing tablet.
14. The display device of claim 13 wherein said piezoelectric
transducer comprises at least one polarized piezoelectric sheet
comprised of piezoelectric polymer and an electrical conducting
layer on each side of the sheet.
15. The display device of claim 14 comprising electrical conductors
extending from said piezoelectric transducer to display
electrically conductive layers disposed on either side of a
cholesteric liquid crystal layer of said writing tablet, said
voltage being applied along said conductors to said display
electrically conductive layers, wherein said piezoelectric
transducer comprises a stack of said polarized piezoelectric sheets
in which each of the piezoelectric sheets is coated with said
electrically conductive layer on the upper and lower side of the
piezoelectric sheet, wherein said electrically conductive layers on
the upper sides of the piezoelectric sheets are electrically
connected together and connected to one of said display electrical
conductors and said electrically conductive layers on the lower
sides of the piezoelectric sheets are electrically connected
together and connected to the other display electrical conductive
layer.
16. A display device comprising a cholesteric liquid crystal
display, a piezoelectric transducer and a device that mechanically
applies a force to said piezoelectric transducer that generates a
voltage that is applied to said display that places said display in
at least one of a color reflective planar state, a substantially
transparent focal conic state and a gray scale state.
17. The display device of claim 16 wherein said device comprises a
spring loaded striker or a snap action switch that applies said
force to said piezoelectric transducer.
18. The display device of claim 16 wherein said liquid crystal
display comprises a writing tablet that is flexible and includes no
rigid housing so as to form liquid crystal paper, wherein said
piezoelectric transducer is integrated with said writing tablet and
said display device includes no power source besides said
piezoelectric transducer.
Description
BACKGROUND OF THE INVENTION
[0001] Recently, the Boogie Board.RTM. pressure sensitive
cholesteric liquid crystal writing tablet, of Improv.RTM.
Electronics has appeared on the market in which a pointed stylus or
the finger nail can be used to write or trace an image on the
surface of the tablet as described in U.S. Pat. No. 6,104,448. This
tablet offers a considerable improvement over previous tablet
technologies in that the image can be simply and instantly erased
with the push of a button that applies a voltage pulse to
electrodes in the tablet. In a cholesteric liquid crystal writing
tablet, the liquid crystal is sandwiched between two substrates
each carrying a transparent electrically conductive layer, which
are spaced to a particular gap. The upper substrate is flexible and
the bottom substrate is painted with a fixed opaque light absorbing
dark background. Within the gap is a bistable cholesteric liquid
crystal dispersed in a polymer network which can exhibit two
textures, an essentially transparent (focal conic) texture and a
color reflective (planar) texture. The spacing of the cell gap is
usually set by plastic or glass spacers that are either cylindrical
or spherical in shape. The transparent conductive layers are
exposed on ledges for connecting to drive electronics so that a
voltage or voltage pulses may be applied across the electrically
conductive layers as is sufficient to initialize or erase an
image.
[0002] The commercially available Boogie Board.RTM. writing tablet
is initialized by applying voltage pulses to the electrodes to
electrically drive the cholesteric material to the focal conic
state. When one presses on the top substrate with a pointed stylus
or finger, the liquid crystal is locally displaced. Flow induced in
the liquid crystal changes its optical texture from essentially
transparent to a brilliant reflective color at the location of the
stylus. The polymer network limits the flow to produce a desired
line width. The reflective color of the traced image contrasts well
with the dark background of the lower substrate. An image traced by
the stylus or finger will remain on the tablet indefinitely without
application of a voltage until erased.
[0003] Erasure is accomplished by applying a voltage pulse to
transparent conducting electrodes on the inner surface of the
substrates that drives the cholesteric liquid crystal from its
color reflective state back to its essentially transparent state.
The required electrical driving pulses are typically provided by
electronics which require a DC voltage supply, such as a battery,
for the operation. Driving schemes for switching cholesteric
displays are described in U.S. Pat. Nos. 5,251,048, 5,644,330,
5,748,277, 5,889,566, 6,133,895 and 7,023,409, all incorporated
herein by reference. All aspects of the cholesteric liquid crystal
writing tablet, including the Boogie Board.RTM. described above,
are suitable for the cholesteric liquid crystal writing tablet used
in the display device of this disclosure described below.
[0004] While the lifetime of a battery in the Boogie Board.RTM.
writing tablet is long, eliminating the battery would be an
improvement since then the device would not need recharging or
replacement of the battery. A more significant feature is in the
cost reduction. When a battery is utilized, circuitry converts the
DC battery voltage into an appropriate pulse or pulse sequence
which adds to the cost of the writing tablet not only in cost of
materials but more significantly in the cost of labor in
manufacturing the writing tablet. It would therefore be
advantageous to have a different erasing means that is free of
batteries with circuitry that is simpler and potentially of lower
cost.
BRIEF DESCRIPTION
[0005] We disclose a display device including a cholesteric liquid
crystal writing tablet that does not require a battery for erasing
the image. The writing tablet is erased by a piezoelectric
transducer. Piezoelectricity is a linear coupling between stress
and electric polarization, discovered in 1880 by Pierre and Jacques
Curie. Materials which exhibit piezoelectricity are organic
materials such as PVDF material described, for example in U.S. Pat.
No. 6,104,119, incorporated herein by reference. Known inorganic
piezoelectric materials include lead zirconate titanate (PZT),
barium titanate (BaTiO.sub.3) and other piezoceramic materials.
Piezoelectric materials have numerous applications in ultrasonics,
hydroacoustics, frequency standards and in ferroelectric ceramics
used in sensors, transducers, vibration dampeners and energy
harvesters. A common use of piezoelectric transducers is in flame
igniters used to ignite outdoor barbeque grills, fire places,
cigarette lighters etc. The output of a piezoelectric transducer is
usually a high voltage pulse following a mechanical impulse applied
to mechanically strain the piezoelectric material. We have
discovered that it is possible to make use of this voltage pulse to
erase a cholesteric liquid crystal writing tablet. Additional
circuitry to shape the waveform of the output signal and couple it
to a cholesteric writing tablet may or may not be used. This
innovative utilization of the piezoelectric transducer for the
cholesteric writing tablet allows for the elimination of batteries
and minimizes or completely eliminates the use of the driving
electronics to reduce the product cost.
[0006] A first embodiment of the disclosure features a display
device comprising a cholesteric liquid crystal writing tablet and a
piezoelectric transducer. The piezoelectric transducer is subjected
to a mechanical force that generates a voltage that is applied to
the writing tablet that erases writing (including drawing and
images) on the writing tablet.
[0007] Referring now to specific features of the first embodiment,
the erasing can be carried out by the voltage placing the writing
tablet or a portion thereof in a focal conic texture. The display
device may be constructed to include no power source (e.g., no
batteries) besides the piezoelectric transducer. Further, a
piezoelectric material of the piezoelectric transducer can comprise
a piezoceramic crystal or piezoelectric polymer. The piezoceramic
crystal can comprise barium titanate or lead zirconate titanate.
The piezoelectric polymer can comprise (polarized) polyvinylidene
fluoride (PVDF) or a (polarized) copolymer thereof. The
piezoelectric material of the piezoelectric transducer can comprise
a complex of piezoelectric powder or piezoelectric particles
dispersed in polymeric binder. The display device can comprise a
"snap-action switch" for providing a mechanical force to the
piezoelectric material resulting in the voltage that erases the
writing on the writing tablet. In another variation, the
piezoelectric transducer can comprise two electrical electrodes
electrically connected to leads of the writing tablet. Electrical
conductors (e.g., electrical wires) can extend from the
piezoelectric transducer (from its electrodes) to display
electrically conductive layers disposed on either side of
cholesteric liquid crystal of the writing tablet (to the leads of
the writing tablet connected to the display conductive layers), the
voltage being applied along the conductors to the display
electrically conductive layers. Any of the display electrically
conductive layers of this disclosure can be continuous (covering
substantially the entire viewing area of the writing tablet or
other liquid crystal display), or may be patterned or
segmented.
[0008] The piezoelectric transducer can comprise a stack of
polarized piezoelectric sheets of piezoelectric polymer in which
each of the piezoelectric sheets is coated with an electrically
conductive layer on the upper and lower side of the piezoelectric
sheet; adjacent electrically conductive layers being alternatively
electrically connected to a different one of the display
electrically conductive layers. Any display device of this
disclosure can comprise a device that mechanically applies a force
to the piezoelectric transducer (e.g., the stack of piezoelectric
polarized polymer sheets or the piezoelectric ceramic) that results
in applying the voltage to the cholesteric liquid crystal display
(e.g., the cholesteric liquid crystal display writing tablet). In
another aspect, the writing tablet is flexible (e.g., and contains
no rigid housing); the piezoelectric transducer comprising the
stack of polarized piezoelectric sheets of piezoelectric polymer is
laminated to the writing tablet on a side opposite an imaging side,
and whereby the mechanical force is achieved by bending the
laminated writing tablet and piezoelectric transducer together
resulting in applying the voltage that erases writing on the
writing tablet.
[0009] Still further, the display device can comprise coupling
circuitry for the transfer of electrical charge from the
piezoelectric transducer to the writing tablet such as may be
suitable for shaping the output signal or for minimizing signal
loss. General design requirements for circuitry coupling a
piezoelectric transducer to various kinds of loads can be found,
for example, in the Piezo Film Technical Manual of SI Images, Inc.,
publically accessible as least as early as Jun. 6, 2013. The
piezoelectric transducer can be subjected to a mechanical force
(e.g., by hand bending or using a mechanical device) that strains
the material that forms the transducer sufficient to generate the
voltage of an amplitude and duration. A mechanical device (e.g., a
spring-loaded hammer or striker) can be used to strike the
piezoelectric transducer (e.g., ceramic piezoelectric) when
operated, to apply the mechanical force and thereby generate a
voltage. The voltage can comprise one or more voltage pulses. There
can be a permanent electrical connection between the piezoelectric
transducer and the writing tablet along which the voltage is
applied.
[0010] Specific features described in the Detailed Description can
be used in the first embodiment, and along with the specific
features described above that are useful in the first embodiment,
in any combination.
[0011] A second embodiment of the disclosure features a display
device comprising a cholesteric liquid crystal display and a
piezoelectric transducer. The piezoelectric transducer is subjected
to a mechanical force that generates a voltage that is applied to
the display that places the display or a portion thereof in at
least one of a color reflective planar state, a substantially
transparent focal conic state and a gray scale state.
[0012] The specific features described in the Detailed Description
and above in connection with the first embodiment, may be used in
connection with the second embodiment, in any combination. It
should be appreciated that reference to a cholesteric liquid
crystal display throughout this disclosure is not limited to a
writing tablet, which is an example of such a display or to an
eBoard which includes two or more such writing tablets, but
encompasses all types of cholesteric liquid crystal displays.
[0013] A third embodiment features a method of erasing the
cholesteric liquid crystal writing tablet as described in the first
embodiment comprising applying a mechanical force that deforms the
piezoelectric transducer so as to apply the voltage to the writing
tablet that erases writing on the writing tablet or a portion
thereof.
[0014] Referring to specific features of the third embodiment, the
erasing can be carried out by the application of the voltage
placing the writing tablet or a portion thereof into a focal conic
texture. There can be a permanent electrical connection between the
piezoelectric transducer and the writing tablet along which the
voltage is applied. The display device can be constructed such that
no other voltage is applied to the writing tablet besides the
voltage applied by the piezoelectric transducer (e.g., no battery
is needed in the display device). The piezoelectric transducer can
comprise two electrical (conducting) electrodes electrically
connected to (electrically conducting) leads of the writing tablet,
and the method applies the voltage from the electrodes to the
leads. Electrical conductors (e.g., wires) can extend from the
piezoelectric transducer (from its leads) to display electrically
conductive layers disposed on either side of the cholesteric liquid
crystal layer of the writing tablet (to the leads of the writing
tablet connected to the display conductive layers), and the voltage
can be applied along the conductors to the display electrically
conductive layers. Coupling circuitry can be used for the transfer
of electrical charge from the piezoelectric transducer to the
writing tablet such as may be suitable for shaping the output
signal or for minimizing signal loss. The application of force or
pressure to the piezoelectric transducer can generate the voltage
of an amplitude and duration. The voltage can comprise one or more
voltage pulses.
[0015] Regarding further specific features of the method, the
method can comprise providing a device that applies the mechanical
force to the piezoelectric transducer that results in application
of the voltage to the writing tablet. The piezoelectric transducer
can comprise a stack of piezoelectric sheets comprising
piezoelectric polymer, electrically connected in parallel and the
method can comprise providing a device that applies the mechanical
force that bends the piezoelectric sheets so as to result in
application of the voltage to the writing tablet. In any aspect of
the method, the deforming can occur by applying a mechanical force
that bends the piezoelectric transducer. In all embodiments of this
disclosure, reference to a mechanical force means a force applied
using a mechanical device or by hand as opposed to other
non-mechanical forces such as electrical force.
[0016] A fourth embodiment features a display device comprising a
cholesteric liquid crystal writing tablet, a piezoelectric
transducer, and a device that mechanically applies a force to the
piezoelectric transducer that generates a voltage that is applied
to the writing tablet that erases writing on the writing
tablet.
[0017] Referring to specific features of the fourth embodiment, the
display device can comprise coupling circuitry for the transfer of
electrical charge from the piezoelectric transducer to the writing
tablet so as to shape an output signal or minimize signal loss from
the piezoelectric transducer. The coupling circuitry can comprise a
diode to rectify an output of the piezoelectric transducer, and the
method includes providing a voltage substantially of one polarity
to the writing tablet. On the other hand, the coupling circuitry
can comprise a resonant inductive and capacitor combination to
shape the output signal of the piezoelectric transducer. Electrical
conductors can extend from the piezoelectric transducer to display
electrically conductive layers disposed on either side of a
cholesteric liquid crystal layer of the writing tablet, the voltage
being applied along the conductors to the display electrically
conductive layers.
[0018] The device can comprise a spring loaded striker that applies
the force to the piezoelectric transducer. In another feature, the
piezoelectric transducer can comprise at least one polarized
piezoelectric sheet comprised of piezoelectric polymer and an
electrical conducting layer on each side of the sheet. The
piezoelectric polymer can comprise polarized polyvinylidene
fluoride or a polarized copolymer thereof. Moreover, the device can
comprise a snap action switch that applies the force to the
piezoelectric transducer that includes the at least one polarized
piezoelectric sheet comprised of piezoelectric polymer. Still
further, a permanent electrical connection can exist between the
piezoelectric transducer and the writing tablet along which the
voltage is applied. Also, the display device can be designed so as
to include no power source besides the piezoelectric
transducer.
[0019] The specific features described in the Detailed Description
and above in connection with the first through third embodiments,
may be used in connection with the fourth embodiment, in any
combination.
[0020] A fifth embodiment features a display device comprising a
cholesteric liquid crystal writing tablet and a piezoelectric
transducer. The writing tablet is flexible and the piezoelectric
transducer is laminated to the writing tablet on a side opposite an
imaging side or side nearest to the viewer. The piezoelectric
transducer is subjected to a mechanical force that is achieved by
bending the laminated writing tablet and piezoelectric transducer
together resulting in the piezoelectric transducer applying a
voltage to the writing tablet that erases writing on the writing
tablet.
[0021] Referring to specific features of the fifth embodiment, the
piezoelectric transducer can comprise at least one polarized
piezoelectric sheet comprised of piezoelectric polymer and an
electrical conducting layer on each side of the sheet. The display
device can comprise electrical conductors extending from the
piezoelectric transducer to display electrically conductive layers
disposed on either side of a cholesteric liquid crystal layer of
the writing tablet; voltage being applied along the conductors to
the display electrically conductive layers; wherein the
piezoelectric transducer comprises a stack of the polarized
piezoelectric sheets in which each of the piezoelectric sheets is
coated with the electrically conductive layer on the upper and
lower side of the piezoelectric sheet; wherein the electrically
conductive layers on the upper sides of the piezoelectric sheets
are electrically connected together and connected to one of the
display electrical conductors and the electrically conductive
layers on the lower sides of the piezoelectric sheets are
electrically connected together and connected to the other display
electrical conductive layer.
[0022] The specific features described in the Detailed Description
and above in connection with the first through fourth embodiments,
may be used in connection with the fifth embodiment, in any
combination.
[0023] A sixth embodiment of a display device comprises a
cholesteric liquid crystal display, a piezoelectric transducer and
a device that mechanically applies a force to the piezoelectric
transducer that generates a voltage that is applied to the display
that places the display in at least one of a color reflective
planar state, a substantially transparent focal conic state and a
gray scale state.
[0024] A specific feature that applies to the sixth embodiment is
that the device can comprise a spring loaded striker or a snap
action switch that applies the force to the piezoelectric
transducer.
[0025] The specific features described in the Detailed Description
and above in connection with the first through fifth embodiments,
may be used in connection with the sixth embodiment, in any
combination.
[0026] An aspect that applies to all embodiments, is that the
cholesteric liquid crystal display (writing tablet) comprises
liquid crystal paper that is erased by voltage applied by the
piezoelectric transducer. Liquid crystal paper is flexible and
normally would include no erase mechanism permanently connected to
it. The liquid crystal paper has the components of the Boogie
Board.RTM. without its housing or erase circuit. The liquid crystal
paper is written on in the same way as a typical cholesteric liquid
crystal writing tablet like the Boogie Board.RTM.. One example of
the liquid crystal paper is described in U.S. patent application
Ser. No. 13/621,367 (Pub. No. US-2013-0070184-A1), entitled "Liquid
Crystal Paper," which is incorporated herein by reference in it s
entirety. It would ordinarily be periodically detachably connected
to a separate erase circuit to erase images on the writing tablet.
The electrodes on the sheet of liquid crystal paper would
ordinarily be placed in direct electrical contact with the
electrodes of the separate erasing circuit providing the voltage
waveforms. However, when the piezoelectric transducer is integrated
into the display device, for example, in the fifth embodiment, or
any of the other embodiments of this disclosure, the liquid crystal
paper could still be flexible like ordinary paper and would not
include any other power source besides the piezoelectric transducer
(e.g., no batteries).
[0027] Many additional features, advantages and a fuller
understanding of the embodiments of the disclosure will be had from
the accompanying drawings and the detailed description that
follows. It should be understood that the above Brief Description
describes embodiments of the disclosure in broad terms while the
following Detailed Description describes embodiments of the
disclosure more narrowly and presents specific embodiments that
should not be construed as necessary limitations of the invention
as broadly defined in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1: Schematic of a display device including a
cholesteric liquid crystal display electrically connected to a
piezoelectric transducer and circuit electronics.
[0029] FIG. 2: Schematic of a display device including a
cholesteric liquid crystal display electrically connected to a
piezoelectric transducer made of flexible PVDF material.
[0030] FIG. 3a: Photograph of a cholesteric liquid crystal display
writing tablet of Example 1 with a written image, which is
electrically connected to a piezoelectric PVDF transducer.
[0031] FIG. 3b: Photograph of the writing tablet of FIG. 3a erased
to the focal conic state after flexing the piezoelectric PVDF
polymer material.
[0032] FIG. 4: Schematic illustrating a display device including an
integrated flexible writing tablet and piezoelectric
transducer.
[0033] FIG. 5: Schematic of a display device including a
cholesteric liquid crystal display electrically connected to a
ceramic piezoelectric igniter transducer and detailed added
circuitry.
[0034] FIG. 6: Schematic of a coupling circuit described in Example
2.
[0035] It should be appreciated that like reference numerals
represent the same or similar parts throughout the several views of
this disclosure.
DETAILED DESCRIPTION
[0036] FIG. 1 shows a schematic of a display device including a
cholesteric liquid crystal display writing tablet 80 connected to a
piezoelectric transducer 50 through optional circuit electronics
55. Transducer 50 can be activated by an optional mechanical device
58, or by hand, which applies a mechanical force that causes the
transducer to generate an electric signal for erasing the display
device. In some cases circuit electronics 55 are desired to shape
the waveform of the electric signal generated by the piezoelectric
transducer and to transfer the electrical signal from the
transducer to the writing tablet. The circuit 55 may be a simple
resistive potential divider or may involve diodes or other
electrical elements (e.g., semiconductor elements). Examples below
are provided for a transducer requiring no coupling circuit and a
different type of transducer with which a coupling circuit is
utilized. Electronic characteristics of piezoelectric transducers
are known in the art as, for example, can be found in the Piezo
Film Technical Manual of SI Images, Inc., publically accessible as
least as early as Jun. 6, 2013, which is incorporated herein by
reference in its entirety. The cholesteric writing tablet 80
includes a top substrate 10 with a conductive layer 20, a bottom
substrate 11 with a conductive layer 21, a light absorbing layer 40
as an outermost layer on one side of the display and cholesteric
liquid crystal layer 30 disposed between the conductive layers 20
and 21. The cholesteric liquid crystal layer 30 includes
cholesteric liquid crystal material dispersed in a polymer matrix.
The conductive layers 20 and 21 are continuous and extend over
substantially the entire viewing area of the display. This
effectively makes the display a single pixel display. All of the
substrates and conductive layers can be transparent, except for
those below the liquid crystal layer which need not be transparent
but should not be reflective.
[0037] The writing tablet could be tiled to other writing tablets
to make a larger device such as an eBoard as disclosed in U.S.
patent application Ser. No. 13/897,004, entitled "Cholesteric
Writing Board Display Device," which is incorporated herein by
reference in its entirety. Each writing tablet of the eBoard could
be erased by a single piezoelectric transducer or possibly multiple
writing tablets could be wired so as to be erased by a single
piezoelectric transducer. The single writing tablet display has
ledges 12 and 13 which support terminals 22 and 23, electrically
connected to display conductive layers 20 and 21 respectively.
Suitable ledges on writing tablets of the eBoard are disclosed in
the Ser. No. 13/897,004 application. Electrically conducting leads
70 and 71, as illustrated in FIG. 1, connect writing tablet 80 to
the optional coupling circuit 55. The output terminal 54 of
transducer 50 is connected to ground whereas output terminal 53 of
transducer 50 is connected to circuit electronics 55.
Alternatively, if coupling circuitry is not desired, the writing
tablet may be connected directly to the transducer. In this case
(not shown in FIG. 1) lead 70 would be connected directly to
terminal 53 with circuit electronics removed.
[0038] FIG. 2 is an illustration of a flexible piezoelectric
transducer 50a made of a sheet of PVDF material. The polarized
fluoropolymer PVDF, polyvinylidene fluoride, has very high
piezoelectric activity and is processed in the form of a thin film
that is flexible, light weight and rugged. A transducer is made by
coating a polarized film on both sides with conducting material
(see for example Piezo Film Technical Manual of SI Images, Inc.,
publically accessible at least by Jun. 6, 2013). When the film is
flexed a charge is generated on the conductors. The amplitude of
the electrical signal is directly proportional to the mechanical
deformation of the piezoelectric material as well as the size of
the film. The films may be stacked on top of one another and
connected in parallel to increase the amplitude. See Example 1 on
the preparation of such a stack. Further, the piezoelectric
transducer (e.g., PVDF stack) may be incorporated in or with a
snap-action device that provides a mechanical force that deforms
the stack and provides a resulting voltage output pulse. FIG. 2
illustrates the application of such a stack of PVDF films for use
as a transducer that provides a voltage signal for erasing a
cholesteric liquid crystal display. In FIG. 2, the piezoelectric
PVDF transducer 50a has output terminals 53 and 54 electrically
connected to conductive layers 60 and 61 on top and bottom surfaces
of the stack respectively. Conductive wires 70 and 71 directly
connect terminal 53 of the piezoelectric transducer with terminal
22 of the writing tablet 80 and transducer terminal 54 with
terminal 23 of the writing tablet 80, respectively. A protective
dielectric coating (i.e., Mylar) 56 and 57 is applied to the top
and bottom conductive layers 60 and 61, respectively. Each
piezoelectric layer 64 is coated with a thin conductive layer 63
that carries charge once the piezoelectric material has been
flexed. These adjacent conductive layers 63 are alternately
connected to conductors 66 and 67, which are electrically connected
to the top and bottom conductive layers 60, 61, respectively. That
is, one conductor 63 is electrically connected to conductor 66
while the next adjacent conductor 63 is electrically connected to
conductor 67, the conductors 66 and 67 not being in direct
electrical contact with each other in the example shown. The
transducer terminals 53 and 54 are connected to the writing tablet
80 through the wires 70 and 71 to the terminals 22 and 23 of the
writing tablet, respectively. In this case coupling electronic
circuitry is not used. The PVDF piezoelectric transducer is
activated by bending it back and forth applying mechanical forces
F.sub.1 and F.sub.2 (using a mechanical actuator or by hand) as
schematically shown in FIG. 2 with the other end of the film being
fixed in place. The force applied to the piezoelectric material
causes deformation of the film resulting in a sequence of the
electric pulses capable of placing the display 80 in the desired
optical state; i.e., the focal conic state, as is suitable to erase
the writing tablet.
[0039] Various types of devices (e.g., represented schematically by
58 in FIG. 1) for mechanically applying a force that deforms (e.g.,
bends) the piezoelectric sheets resulting in application of the
voltage to the writing tablet may be suitable for use in this
disclosure, such as those described in the Piezo Film Technical
Manual of SI Images, Inc. reported here and in U.S. Pat. No.
6,104,119, which is incorporated herein by reference in its
entirety. One such example device is a snap-action device in which
the piezoelectric sheets are laminated onto a substrate that
quickly snaps from one position to another when pressed. Such
devices are known as snap-action piezoelectric switches.
Snap-action piezoelectric switches typically include a dome-shaped
snap disc to which the piezoelectric material is attached, and a
switch button that is pressed by the user. When the switch button
is pressed, the switch button deforms the snap disc within the
switch and causes it to snap from one position to another.
[0040] Other example piezoelectric devices (e.g., represented
schematically by 58 in FIG. 1) suitable for use as a transducer for
erasing cholesteric displays include a ceramic piezoelectric stack
and a mechanical striker (e.g., a spring-loaded hammer) that
strikes the piezoelectric stack when operated, to apply a
mechanical force and thereby generate a voltage. The voltage
generated by the piezoelectric stack when hit by the striker is
used to erase the display. A mechanical striker can also be used to
deform (e.g., bend) piezoelectric sheets, to thereby apply a
voltage to erase the display.
[0041] A prototype of a cholesteric writing tablet using a PVDF
piezoelectric transducer is described in Example 1.
[0042] An advantage of the PVDF piezoelectric transducer is its
flexibility and its thin film profile. These features allow for its
lamination on the reverse side (side opposite the imaging side) of
the writing tablet display. The writing tablet display is also
flexible since it is made from thin flexible polymer substrates.
The commercial Boogie Board.RTM. cholesteric liquid crystal writing
table display is mounted on a rigid backing or housing; however, a
rigid backing is unnecessary for operation as a display of this
disclosure. Capitalizing on these features, another embodiment for
a piezoelectric erased writing tablet is an integrated version
where the flexible PVDF piezoelectric transducer is laminated
directly on the flexible writing tablet display without the rigid
backing. An integrated device is illustrated in FIG. 4 where a
double stacked PVDF transducer 50c is laminated to the non-viewing
(back) side of the cholesteric liquid crystal display writing
tablet 80 with an inner layer of adhesive 91 disposed between the
stacked PVDF transducer 50c and the writing tablet 80. The PVDF
double stack transducer includes two polarized PVDF layers 64
separated by an electrically conducting layer 63 which is connected
to output terminal 54. The electrically conducting layer electrodes
located on the top and bottom of the stack, 61 and 61a,
respectively, are electrically connected together by electrical
conductor 67, which are further connected to output terminal 53. It
is to be appreciated that the PVDF transducer is not necessarily
limited to a double stack but could, for example, consist of 5
stacked layers of PVDF film with electrical conductors on each side
as illustrated in the Exploded view of transducer 50a in FIG. 2.
Further, the transducer could consist of any number of such PVDF
layers and associated conductor layers as is suitable to implement
a working integrated device. Lead 70 electrically connects
transducer terminal 53 to terminal 22 of the writing tablet and
lead 71 electrically connects transducer output terminal 54 to
terminal 23 of the writing tablet. When the entire construction of
the integrated device (writing tablet and laminated PVDF
transducer) is flexed so as to apply a mechanical force that bends
the piezoelectric transducer 50c, charge generated by the PVDF
transducer is applied to the interconnecting terminals 22 and 23 of
the writing tablet and erases any writing (including drawing and
images) that may have been put on the display by a stylus. FIG. 4
shows the transducer 50c directly coupled to the writing tablet 80.
However, if one desires more effective coupling, circuit
electronics may be added as illustrated in FIG. 1. In this case,
the electronic circuit 55 could be developed with state of the art
electronic knowledge based upon the known electrical
characteristics of both the transducer and the display.
EXAMPLES
Example 1
[0043] A reflective cholesteric liquid crystal display writing
tablet 80 with the size of active area 38 mm.times.76 mm shown in
FIG. 2 was made by forming a cholesteric liquid crystal layer
(cholesteric liquid crystal material dispersed in a polymer matrix)
by a PIPS technique described in U.S. Pat. No. 6,104,448, U.S.
patent application Ser. Nos. 12/152,729 and 12/220,805, which are
all incorporated herein by reference in their entireties, and
disposed between a 5 mil PET top substrate and a 7 mil PET bottom
substrate with conductive polymer layers on one side of both
substrates as illustrated in FIGS. 1 and 2. As illustrated in the
exploded view of FIG. 2, a PVDF piezoelectric transducer 50a was
constructed by stacking 20 strips of PVDF material PZ-02 (6
mm.times.41 mm.times.0.2 mm size) 64 purchased from Image SI, Inc.
The adjacent conducting layers 63 are alternatively electrically
connected to conductors 66 and 67, which are electrically connected
to the top and bottom conductive layers 60, 61, respectively. The
conductors 60, 61 are connected to the display 80 through the wires
70 and 71 to the terminals 22 and 23 of the writing tablet 80,
respectively. A pointed stylus was used to write an image 90 on the
writing tablet 80 as shown in the photograph of FIG. 3a (the bright
written image 90 being in the planar texture and the dark areas of
the display being in the focal conic texture). Depending on the
amplitude and frequency of the applied bending force, the PVDF
stack produced from 10V to 35V voltage pulses with a pulse width
from 100 ms to 300 ms. After bending the stack 50a back and forth
four times by hand such voltage pulses were achieved and thus, the
writing 90 on the display 80 was erased by placing the liquid
crystal of the entire viewing area in the focal conic texture as
shown in photograph--FIG. 3b.
Example 2
[0044] A piezoelectric transducer from a commercial multipurpose
piezoelectric flame igniter was used to erase a cholesteric liquid
crystal display writing tablet. The writing tablet for this example
was obtained from a commercial Boogie Board.RTM. product of Improv
Electronics. The display from the Boogie Board.RTM. was cut with a
pair of scissors to a size of 1 inch.times.3 inches while keeping
the two electrode terminals as part of the display. The resulting
display 80 is illustrated schematically in FIG. 5. The piezo
igniter 50b was extracted from a Click n Flame.TM. multipurpose
lighter. The multipurpose lighter was disassembled and the piezo
igniter, including a piezoelectric transducer and a "spring loaded
striker" to provide a mechanical force (impulse) to the
piezoelectric transducer, were taken from the lighter along with
the connecting electrical leads. As illustrated in FIG. 5, one of
the electrical leads from the terminal 53 of transducer 50b is
connected to a 100 k.OMEGA. resistor 52 in series with one of the
writing tablet electrode terminals 22 of the writing tablet 80. The
other lead from the transducer terminal 54 is connected directly to
the remaining terminal 23 of the writing tablet. A diode 51 is
connected in parallel with the terminals 22 and 23 of the writing
tablet 80. The resulting circuit 55 served to shape the pulse so as
to transfer charge from the transducer to the display. When the
trigger on the "spring loaded striker" device of the igniter
transducer 50b was compressed, the piezo igniter produced a spiked
electrical pulse with a maximum amplitude >200V decaying
approximately exponentially to 0 V over a period of 2.5 ms as
measured across the diode. An image was written on the display with
the slight pressure of a pointed stylus (bright planar writing with
surrounding areas in the focal conic texture appearing dark). The
image was erased and the entire viewing area was placed in the
focal conic state after compressing the trigger of the piezo
igniter four consecutive times.
[0045] In Example 2 above the coupling circuitry or the electronic
circuit 55 was used to rectify the output of the piezoelectric
transducer which was a piezo igniter 50b extracted from a Click n
Flame.TM. multipurpose lighter. The diode provided a voltage
substantially of one polarity to the writing tablet to erase
written images. It is well known in the art (see for example U.S.
Pat. No. 6,104,448) that AC voltages may also be used. FIG. 6 is
another possible circuit diagram for circuit electronics 55
including an inductor 100 and capacitor 102 for shaping or
conditioning the output signal from a transducer terminal 53 (see
FIG. 1) for erasing a written image on the cholesteric liquid
crystal writing tablet. Electrical conducting lead 70 connects the
circuit to the writing tablet. The values of the inductor 100 and
capacitor 102 can be selected to provide resonance frequency less
than 2.0 kHz as determined by the parallel resonance condition
1/2.pi.(LC).sup.1/2 where L and C are the inductance and
capacitance of the inductor 100 and capacitor 102 respectively. The
upper limit of 2.0 kHz is established by the response
characteristic of the writing tablet of the size of the commercial
Boogie Board.RTM. of Improv Electronics, but could be higher for
smaller sizes or lower for larger sizes. The selection of
appropriate values of the inductor and capacitor would be
appreciated by one skilled in the art in view of this disclosure.
While this circuit has not been tested experimentally, the concept
of the resonance circuit is to provide an AC signal to the writing
tablet at a frequency sufficient to erase a written image. The
optimum frequency is controlled by the effective resistance and
capacitance of the writing tablet as affected by the tablet's
size.
[0046] Many modifications and variations of the disclosed
embodiments will be apparent to those of ordinary skill in the art
in light of the foregoing disclosure. Therefore, it is to be
understood that, within the scope of the appended claims, the
invention can be practiced otherwise than has been specifically
shown and described.
* * * * *